This invention relates to maintaining a unity power factor in a power supply.
Published German Application No. 37 38 180 A1 discloses one circuit for maintaining a unity power factor in a device having a PWM-controlled power converter supplied through filter inductors from an AC main terminal. Power semiconductor switch devices are employed for delivering at a direct current link a voltage Ud across a filter capacitor for supply of the load connected thereto. A signal corresponding to the voltage control deviation is used as a current amplitude signal at the output of a voltage regulator, and is fed to a first input terminal of a multiplication circuit. A second input to the multiplication circuit is the main AC voltage signal, the multiplication of the two resulting in a current reference for each phase at the output of the multipliers. That output is compared with actual AC input current signal, and that output is delivered for operating switch devices in the power converter.
With such a circuit, it is assured that the current received from the AC main is of substantially sinusoidal waveform and is in phase with the AC main voltage, even if the load is inductive, thereby decreasing power loss and improving the power factor. The closer that the power factor is to one, the less the power loss.
During certain motor operations, such as braking, the motor runs as a generator. Therefore it is necessary to dissipate energy such as by means of a dynamic brake. One object of the present invention is to avoid the necessity of purchasing dissipator dynamic brakes or other means to dissipate inertial energy and to avoid large, costly magnetic coils to control harmonics. A control circuit is provided which allows mechanical energy from the motor shift side to be returned to the main power supply and still maintain a unity power factor and a sinusoidal current waveform.